GALFACTS RFI Excision Methods

Andrecut, Mircea

Radio astronomical observations are susceptible to Radio Frequency Interference (RFI) contamination. In this case, the signal from astrophysical sources is distorted due to close, and relatively strong, radio emissions from other sources operating on the same frequency spectrum (communication services, for example). As a consequence, the observation data needs to be cleaned, by removing the undesired RFI components, while preserving as much of the underlying useful information as possible. As the data acquisition rates of radio telescopes increases and observations bandwidth extend beyond protected spectral allocations, software systems to mitigate RFI signals are becoming critical. Here, we discuss the RFI excision methods implemented in the data processing pipeline of the Galactic ALFA Continuum Survey (GALFACTS), which is a large-area spectro-polarimetric survey being carried out with the Arecibo Radio telescope in Puerto Rico. The implemented algorithms are run in both frequency and time domains, and they use an adaptive thresholding strategy to detect and reject the RFI outliers. Detection of an RFI signal will result on a flag bit being set to identify bad data points. The search is done first in the frequency domain, which flags bad channels for each time step. Then it is done in the time domain to flag bad time samples. In this case all the channels for a given time sample are flagged.